This invention relates to digital speech synthesis circuits capable of being implemented in an integrated circuit device. More specifically, this invention relates to a speech synthesis system in which digital speech data may be inputted as prearranged coded values or may be directly loaded into an input register, with the coded or uncoded format of the digital speech data being detectable to enable decoding circuitry within the speech synthesis system to decode coded digital speech data such that all calculations within the system are based upon uncoded digital speech parameters.
Several techniques are known in the prior art for digitizing human speech. For example, pulse code modulation, differential pulse code modulation, adaptive predictive coding, delta modulation, channel vocoders, cepstrum vocoders, formant vocoders, voice excited vocoders, and linear predictive coding techniques of speech digitization are known. The techniques are briefly explained in "Voice Signals; Bit by Bit" on pages 28-34 of the October, 1973 issue of IEEE Spectrum.
In certain applications and particularly those in which digitized speech is to be stored in a memory, most researchers tend to use the linear predictive coding technique because it produces a very high quality speech using rather low data rates. An excellent example of the use of linear predictive coding systems, implementable in integrated circuit techniques may be seen in U.S. patent application Ser. No. 901,393, filed Apr. 28, 1978, now U.S. Pat. No. 4,209,836 issued June 24, 1980. The speech synthesis system described in the aforementioned U.S. Pat. No. 4,209,836 utilizes frames of data which are comprised of digital representations of pitch, energy and certain linear predictive coefficients which are utilized to control a digital filter. The system described in the aforementioned U.S. Pat. No. 4,209,836 is capable of producing high quality synthetic human speech at a bit rate of as low as 1200 bits per second, utilizing a fixed rate of data frame entry. The speech synthesis system of U.S. Pat. No. 4,209,836 utilizes a coded input parameter system common in speech synthesis systems wherein a coded value is utilized to minimize the date input rate required by the speech synthesis system. Such coded parameters may accurately portray the values of parameters necessary to synthesize human speech; however, certain speech parameter generation algorithms provide the necessary filter, pitch, and energy parameters in uncoded format. While an increase in the bit rate required to accurately synthesize human speech is experienced while utilizing uncoded parameters, the coding method may add unnecessary steps to a system wherein filter parameters are directly calculated. A speech synthesis system capable of utilizing both coded and uncoded parameters to generate human speech must be capable of accepting input parameters of various lengths, since coded parameters normally consist of fewer bits than uncoded parameters.
It is therefore one object of this invention to improve speech synthesis technology.
It is another object of this invention to provide a speech synthesis system capable of accurately synthesizing human speech from input parameters which are directly loaded or loaded in an encoded format.
It is still another object of this invention to provide a speech synthesis system capable of synthesizing human speech from frames of data which may consist of parameters of varying lengths.
The foregoing objects are achieved as is now described. A speech synthesis system implementable in an integrated circuit device and capable of converting frames of data into analog signals representative of human speech is constructed. The frames of data are comprised of digital representations of values of pitch, energy and certain filter coefficients. The binary representations of pitch, energy and filter coefficients may be inputted as prearranged coded values or may be directly loaded into a data input register. Control circuitry within the speech synthesis system detects a control signal indicating whether the parameters being inputted are in coded or uncoded format, and provides decoding circuitry within the speech synthesis system. All calculations within the speech synthesis system are thus based upon uncoded input parameters.